Think for a second. Castform is the Weather Pokemon.
If it could become Grass, Electric and Fairy types from the new terrain moves, Castform would start to become a lot like Kecleon/Arceus.
Nintendo most likely wanted to keep the idea of a unique Pokemon that changes in various natural weather conditions. But why not sandstorm? Because Sandstorms are caused by wind, and come from dust on the ground, and not the sky. (which is what Game Freak is appearing to go for, transforming from weather that comes from the sky.) If Castform transformed into Fairy Type from Fairy Terrain, it wouldn't be as unique, as Terrain ≠ Weather.
Also, just because a Pokemon is in an egg group doesn't mean they are of that type. For example, Delibird is in Water 1. But, surprise, it's not a water type! :O